Like salmon? Pop fish oil[4]? Got a prostate? Then listen up. A new cancer study[5] rolled in this week, and at first glance, it looks like bad news for any fish-loving men out there. A team of researchers at the Fred Hutchinson Cancer Research Center found a disturbing link between blood levels of DHA – that darling omega-3 fat abundant in seafood – and the risk of developing aggressive, “high-grade” prostate tumors.

The eyebrow-raising results of the study showed that:

Compared to folks with low DHA levels, men with the highest levels were 2.5 times more likely to wind up with aggressively growing prostate tumors.

Men with the highest blood levels of trans fatty acids[6] were the least likely to get aggressive prostate cancer. (Say what?)

According to the researchers, very few of the study subjects were supplementing with fish oil – so these findings probably don’t reflect unbridled mega-dosing of omega-3 fats. Despite its consistent role in boosting heart-health, it looks like fish oil could be a double-edged sword. And maybe a stick of trans-fatty margarine is really a cancer cure. Intriguing, eh?

Something’s Fishy

The researchers weren’t shy about expressing their surprise. Team leader Theodore Brasky stated[7] that he and his colleagues were “stunned to see these results,” and the article’s full-text discussion notes that the findings were the exact opposite of what the team expected.

To top that off, this study derails the otherwise consistent train of research showing that DHA seems protective at best (and neutral at worst) when it comes to prostate health. In 2001, a study of over 6,000 Swedish men[8] found that the folks eating the most fish had drastically lower rates of prostate cancer than those eating the least. Another study from New Zealand[9] found that men with the highest DHA markers slashed their prostate cancer risk by 38% compared to the men with the lowest DHA levels. And yet another study tracking Japanese men[10] in Japan and Brazil found that omega-3 levels in the blood were consistently linked with a reduction in prostate cancer.

Obviously, fish is only bad for you if you’re American.

For the science savvy, there are even some biological pathways[11] explaining how DHA could lower prostate cancer risk. Which brings us to another predicament: We haven’t sleuthed out any mechanism that could explain why DHA (but not other polyunsaturated fats[12]) promotes rapid tumor growth. Nor do we know of any way trans fats could save a prostate from malignant doom. This is a classic case of correlation clashing with biological plausibility – and it highlights why observational studies, with their slew of undocumented variables and contradictory findings, can’t tell us anything definitive about food and disease.

But that doesn’t mean this study is worth ignoring – just that it might warrant an extra dollop of scrutiny. Let’s take a look at exactly what these researchers uncovered, and see whether fish is really off the proverbial hook.

DHA in Your Blood Versus DHA in Your Mouth

First things first: Can we really conclude that the men with high DHA levels were eating more fish (or fish oil) than the rest of the gang?

The answer is a resounding “nope” – and this is why the current fish-condemning media coverage[13] is a load of hooey. Although consuming more DHA can definitely boost your serum levels, the reverse isn’t always true: elevated DHA doesn’t automatically mean you’re a sushi fiend or loyal fish-oil guzzler. Guess what else can increase omega-3 fats in your blood? You’ll get a hoot from this one: low-fat diets.

Check out this intervention study from 2001[14], which measured changes is serum fatty acids after feeding folks either a low fat (20% of calories) or high fat (45% of calories) diet. Although the low-fat dieters didn’t get any special omega-3 boost, the levels in their blood rose disproportionately by the end of the trial. The study concluded that “Consumption of a low fat diet alters fatty acid patterns in a manner similar to that observed with feeding of (n-3) long-chain fatty acids.” In other words, fat restriction caused blood levels of omega-3 fats to resemble that of seafood lovers.

So what does this mean for our current study? For starters, if there is a legitimate link between high DHA levels in the blood and aggressive prostate cancer, we could point the finger at low-fat diets just as easily as fish. Is dietary DHA a trigger for tumor growth, or is it some consequence of fat reduction – such as the higher intake of grains or sugar that often fills the caloric void? Since this study didn’t document any diet variables, there’s no way to tell for sure. But what we do know is fish can’t be charged guilty by this particular chunk of research.

More to the Story

One final problem with this study is that it only documented a handful of the fatty acids present in the blood. Here’s a low-carb pie chart for your viewing pleasure:

The labeled slices show the eight types of fatty acids this study examined. But see that big green Mystery Slice? It’s the placeholder for the all the saturated and monounsaturated forms the study didn’t report. That’s a whole lotta’ mysteriousness. And because the researchers looked at fatty acid levels as percentages rather than absolute concentrations (a method that may have some major limitations[15] in studies like these), it means that whenever the proportion of one fatty acid rises, at least one of the others has to drop. The higher DHA levels in the aggressive-tumor-ridden men had to go hand-in-hand with lower levels of a different fat. It’s not beyond the realm of possibility that whatever’s going on in the Mystery Slice plays more of a role in tumor growth than DHA.

Not Out of the Water Yet

Even though this study doesn’t give any reason to shun fatty fish or their oils, it’s a nice segue into a related issue: A bad diet plus fish oil is still a bad diet. And given the oxidation-prone nature[12] of all polyunsaturated fats, a massive intake of omega-3’s – despite their brilliance in moderation – could potentially do more harm than good. If you’re trying to restore a healthy ratio of omega fats, avoiding omega-6-rich foods (and supplementing wisely) is a better strategy than chugging fish oil like a frat boy with gin.

All in all, this study is just another drop in the sea of misinterpreted nutritional research. Don’t take the bait!